A. Field of the Invention
This invention relates to a substrate for perpendicular magnetic recording media mounted in a fixed magnetic recording device (hard disk device), which is an external storage device for computers or for consumer equipment. More specifically, the substrate for perpendicular magnetic recording media of this invention is a perpendicular magnetic recording media substrate enabling perpendicular magnetic recording media with superior performance and quality. This invention also relates to perpendicular magnetic recording media using such a substrate.
B. Description of the Related Art
Magnetic disk devices, which are advancing toward markedly higher recording densities even while costs decline, play a central role as external memory devices for computers, and in recent years have also begun to be mounted in other digital consumer equipment as well. In addition, magnetic disk devices are becoming smaller, and are beginning to be adopted in portable music playback devices and other products as well.
Longitudinal recording methods, in which the magnetization easy axis of the magnetic recording layer is oriented parallel to the substrate surface in the magnetic recording media of the magnetic disk device, have been adopted in the prior art. In recent years, in order to attain still higher recording densities, a perpendicular recording method has been adopted in which the magnetization easy axis of the magnetic recording layer is oriented perpendicularly to the substrate surface. In this perpendicular recording method, the magnetization in adjacent regions is aligned in a direction perpendicular to the recording plane, so that the magnetization is stable in magnetization reversal regions even at high recording densities, and excellent thermal fluctuation characteristics and noise characteristics can be realized.
In perpendicular magnetic recording media, normally a backing layer of soft magnetic material is positioned between the substrate and the magnetic recording layer. In perpendicular magnetic recording media, writing of information is achieved by passing the leakage flux from a single magnetic pole head perpendicularly through the substrate plane.
In such perpendicular magnetic recording media, the quality of read signals depends on the perpendicular orientation of the magnetization easy axis in the magnetic recording layer. When this perpendicular orientation is poor, the leakage magnetic flux from the magnetic recording layer is inclined with respect to the substrate plane, so that media noise is increased and the S/N characteristic declines.
The orientation dispersion angle (Δθ50), which indicates the crystal plane inclination distribution of the layer of interest (for example, the magnetic recording layer) in the magnetic recording media, is an index indicating the perpendicular orientation of the magnetization easy axis. This orientation dispersion angle (Δθ50) is defined as the peak half-maximum width of the rocking curve obtained in X-ray diffraction for a specific orientation plane in the target layer, and corresponds to the distribution center of the angle made by the horizontal plane and the specific orientation plane. In order to improve the S/N characteristics, the orientation dispersion angle (Δθ50) of the magnetic recording layer must be decreased.
The orientation dispersion angle (Δθ50) of the magnetic recording layer depends on the orientation dispersion angle (Δθ50) of the orientation control layer positioned directly below the magnetic recording layer in the magnetic recording media. Further, the orientation dispersion angle (Δθ50) of the orientation control layer depends on the surface shape of the substrate positioned below the orientation control layer. In recent years, methods have been disclosed for controlling the orientation dispersion angle (Δθ50) of the orientation control layer, which effects the orientation dispersion angle (Δθ50) of the magnetic recording layer, through the surface roughness (Ra) of the substrate, which is one parameter related to substrate shape.
In Japanese Patent Application Laid-open No. 2006-286029 (corresponding to U.S. Patent No. 2006 222908 and to Chinese Patent No. CN1841513A), a perpendicular magnetic disk device is disclosed comprising perpendicular magnetic recording media having a nonmagnetic substrate, the surface roughness (Ra) of which is 0.35 nm or lower, a soft magnetic layer, an intermediate nonmagnetic layer the perpendicular orientation (Δθ50) of which is 4° or less, and a perpendicular recording layer formed from magnetic material exhibiting perpendicular anisotropy; and a magnetic head, having a write head, with a main magnetic pole, return yoke, and exciting coil, and a magnetoresistance effect read head; in this device, the flying height f of the magnetic head and the average roughness (Ra) of the perpendicular magnetic recording media surface satisfy the relation f>0.61Ra2−3.7Ra+5.9.
In Japanese Patent Application Laid-open No. 2007-26536 (corresponding to International Patent Application No. WO2007/010908A1), a magnetic recording medium is disclosed in which are provided, at least, a soft magnetic backing layer of a soft magnetic material, an orientation control film which controls the orientation of the film immediately above, a perpendicular magnetic film, the magnetization easy axis of which is oriented primarily perpendicularly to the substrate, and a protective film, and in which the magnetic anisotropy ratio (Hmr/Hmc) of the soft magnetic backing layer is 1 or less, and moreover the orientation dispersion angle (Δθ50) is from 1 to 6°.
In “Influence of Substrate Surface Shape at C-axis Distribution in Perpendicular Media”, Masaru Ono et al., Yamagata Fujitsu Ltd., Dig. 31st Annual Conf. Magn. Soc. Jpn. (2007), p. 264, experiments are disclosed in which, after using DC magnetron sputtering to form an FeCo alloy soft magnetic backing layer, Ru intermediate layer, and CoCrPt alloy magnetic layer in order on substrates on which oxide abrasives and diamond abrasives had been employed, CVD was used to form a carbon protective layer, AFM was employed to measure the substrate surface shape in a 1 μm×1 μm field, the crystal orientation was evaluated by the rocking curve method using XRD, and read/write characteristics were evaluated using a 130 Gb/in2-equivalent perpendicular TuMR write head. According to these experimental results, it is reported, there is a deviation among the substrates in the correlation between the calculated average roughness Ra of the substrate surface and the crystal orientation dispersion Δθ50 of the [002] plane of Ru, which is the intermediate layer. Further, with respect to the inclination angle slope obtained by calculating the slope at various locations on the substrate and averaging and the crystal orientation dispersion Δθ50 of Ru, it is reported that good correlation is observed between the inclination angle slope and the crystal orientation dispersion Δθ50 regardless of the abrasive material, and that the crystal orientation of the Ru intermediate layer is greatly influenced by the inclination from the horizontal plane of the substrate surface.
Thus there have been numerous disclosures of technology to control the orientation dispersion angle (Δθ50) of the orientation control layer by means of the substrate surface roughness (Ra). However, as is corroborated by “Influence of Substrate Surface Shape at C-axis Distribution in Perpendicular Media”, Masaru Ono et al., Yamagata Fujitsu Ltd., Dig. 31st Annual Conf. Magn. Soc. Jpn. (2007), p. 264 and in the Embodiments section below, it has been clarified that the relation between the substrate surface roughness (Ra) and the orientation dispersion angle (Δθ50) of the orientation control layer or of the magnetic recording layer differs depending on the final machining method used for the substrate surface. For example, as described in the Embodiments section below, it has been clarified that the orientation dispersion angle (Δθ50) of the magnetic recording layer differs, even for the same surface roughness (Ra), for a case of finish-polishing of the substrate using a double side polisher with a foam urethane pad affixed and a colloidal silica abrasive liquid, and for a case of etching of the substrate using hydrofluoric acid. Hence the substrate surface roughness (Ra) cannot be regarded as a parameter of the substrate shape which enabled adequate control of the orientation dispersion angle (Δθ50) of the orientation control layer or magnetic recording layer.
On the other hand, as disclosed in “Influence of Substrate Surface Shape at C-axis Distribution in Perpendicular Media”, Masaru Ono et al., Yamagata Fujitsu Ltd., Dig. 31st Annual Conf. Magn. Soc. Jpn. (2007), p. 264, there is a good correlation, regardless of the polishing material, between the inclination angle obtained by averaging the calculated inclination at different locations of the substrate, and the crystal orientation dispersion Δθ50 of the orientation control layer (in Japanese Patent Application Laid-open No. 2006-286029, the intermediate layer). Hence there is a need to develop technology in which an appropriate range is set for this inclination angle, or for a parameter relating to the substrate shape in relation to this inclination angle, in order to improve the orientation dispersion angle (Δθ50) of the orientation control layer and magnetic recording layer regardless of the final machining method used on the substrate.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.